Introduction Previous studies have suggested that singing training (ST) has been proposed as an engaging adjunct to pulmonary rehabilitation (PR) patients with COPD and may improve diaphragmatic function and alleviate symptoms. We aimed to explore whether ST (a structured, facilitator-led group singing-based intervention for COPD) was associated with improvements in diaphragmatic function and related clinical outcomes—including pulmonary function, exercise capacity, health-related quality of life, and psychological symptoms. We hypothesized that ST would improve diaphragmatic function and yield better patient-reported outcomes than health education (HE). Methods In this randomized controlled trial (No. ChiCTR2100052874), we conducted with 40 stable COPD patients randomly assigned to either the ST group (n = 20) or the HE group (n = 20). Primary outcomes were ultrasound-derived diaphragmatic function (dynamic and static diaphragmatic excursion and diaphragmatic thickening fraction). Secondary outcomes included respiratory muscle strength (MIP/MEP), pulmonary function (FEV 1 /FVC, FEV 1 predict), exercise capacity (6-min walk distance, 6MWD), health-related quality of life (St George’s Respiratory Questionnaire, SGRQ), and psychological symptoms (Hospital Anxiety and Depression Scale, HADS). We report between-group differences in change (95% CIs) and MCID attainment for 6MWD (≥30 m increase), CAT (≥2-point decrease), SGRQ (≥4-point decrease), and HADS (≥1.5-point decrease); for other outcomes, a pragmatic ≥10% improvement threshold was applied. Results Thirty-three patients completed follow-up (ST n = 18; HE n = 15). Compared with HE, ST produced greater improvement in dynamic diaphragmatic mobility (mean SD change 0.7 ± 0.6 vs. 0.1 ± 0.5 cm; difference 0.6 cm, 95% CI 0.1–1.3; P = 0.03) and diaphragmatic thickening fraction (DTF; 23.4% ± 39.8% vs. 1.4% ± 35.7%; difference 22.0%, 95% CI −3.4–47.3; P = 0.06). For key secondary outcomes, ST showed greater improvements in 6MWD (52 ± 49 vs. 5 ± 46 m; difference 47 m, 95% CI 18–76; P 0.01), SGRQ (−12.6 ± 11.0 vs. −1.8 ± 9.2; difference −10.8, 95% CI −16.8 to −4.8; P 0.01), and HADS-D (−1.6 ± 1.7 vs. +0.7 ± 2.1; difference −2.3, 95% CI −3.5 to −1.0; P 0.01), whereas no between-group differences were observed for FEV1/FVC or FEV1%pred (both P 0.30). Applying a pragmatic ≥10% improvement threshold for diaphragmatic ultrasound outcomes, clinically relevant improvement was achieved by 9/18 (50.0%) vs. 1/15 (6.7%) for dynamic mobility and 13/18 (72.2%) vs. 6/15 (40.0%) for DTF (ST vs. HE). Conclusion In this pilot trial, ST was associated with improved diaphragmatic mobility and several clinically relevant outcomes compared with HE. These findings suggest that ST may serve as a relevant adjunct to pulmonary rehabilitation, however larger trials are needed to confirm efficacy. Clinical Trial Registration https://www.chictr.org.cn/showproj.html?proj=135209 .
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